Load in with no sound and death lag

I've had this happen a few times in the past 2 or 3 months. I load in with no sound at all then almost on a clock my game starts to lag around the 6 minute mark and becomes unplayable by the 10-12 minute mark. Then if you try to tab out or close the window to restart it just becomes a black screen and all your other programs are stuck behind the game window. So I then have to sign out and sign back into my computer, restart league and after that its just fine.